A slow host is one that requires more than a few seconds to accept delivery of a modestly sized email message. To illustrate, consider the following example produced by a verbose transaction of sending email to such a slow host:
/usr/sbin/sendmail -v -Rslowhost.com -qRunning /var/spool/mqueues/q.2/df/f0DHnvO02567 (sequence 1 of 1) firstname.lastname@example.org... Connecting to mx.slowhost.com. via esmtp... 220 mx.slowhost.com ESMTP Sendmail 8.10.1/8.10.1; Fri, 13 Dec 2002 10:50:20 -0700 (MST) >>> EHLO mx.slowhost.com 250-mx.slowhost.com Hello email@example.com [123.45.678.9], pleased to meet you 250-ENHANCEDSTATUSCODES 250-8BITMIME 250-SIZE 250-DSN 250-ONEX 250-ETRN 250-XUSR 250 HELP >>> MAIL From:<firstname.lastname@example.org> SIZE=16 ... You wait 2 minutes for slowhost to look you up. ...
This situation can get worse, especially if the slow site runs slow antispam software, because that site can take 9 or 10 minutes to validate you. This can cause sendmail to seem to hang for 9 or 10 minutes, then suddenly to continue with:
250 2.1.0 <email@example.com>... Sender ok >>> RCPT To:<firstname.lastname@example.org> 250 2.1.5 <email@example.com>... Recipient ok >>> DATA 354 Enter mail, end with "." on a line by itself >>> .
Furthermore, some mail transfer agents (MTAs) start to place a message on disk only after all the data has been received, so writing to an NFS-mounted disk can appear to hang for several seconds:
250 2.0.0 f0DHoNh91321 Message accepted for delivery firstname.lastname@example.org... Sent (f0DHoNh91321 ...